This invention relates to crosslinked hydrophilic resins, methods for preparing crosslinked hydrophilic resins and absorbent structures incorporating the crosslinked hydrophilic resins.
Hydrophilic resins (also referred to as superabsorbent polymers) are primarily used in personal care products which absorb body fluids, for example baby diapers, adult incontinence products, feminine hygiene products, and the like. In such applications, hydrophilic resin particles are incorporated into absorbent structures which contain for example, synthetic and natural fiber or paper based woven and nonwoven structures, and toughened masses of fibers, such as fluff pads. The materials used in such structures can instantaneously absorb aqueous fluids and distribute them over the whole absorbent structure. The structures, in the absence of hydrophilic resin particles, have limited absorption capacity, and are bulky due to the large amount of material needed to provide acceptable absorption capacity and do not retain fluid under pressure. A means for improving the absorbency and fluid retention characteristics of such absorbent structures is to incorporate hydrophilic resin particles which imbibe fluids to form a swollen hydrogel material, see U.S. Pat. No. 4,610,678 (relevant portions incorporated herein by reference). The hydrophilic resin particles quickly absorb fluids and retain such fluids to prevent leakage and give the absorbent structure a "dry feel" even when wetted.
When the water-absorption capacity of hydrophilic resins is increased, the fraction of water-soluble polymer present in the resin is generally increased. At the same time the gel strength of the swollen gel, its absorption capacity under pressure and absorption speed are reduced. The water absorbent resin tends to agglomerate upon wetting which results in a reduction of its absorption capacity and in gel blockage which prevents transport of fluids within the absorbent structure. This is a particular problem when a large fraction of the absorbent structure is replaced with hydrophilic resin particles to prepare a thin absorbent device.
One method of improving the absorption characteristics is to incorporate a compound which crosslinks the final product into the monomer mixture used to prepare the hydrophilic resin. Brandt U.S. Pat. No. 4,654,039 discloses the preparation of hydrophilic resins and known crosslinking agents for such resins, Column 6 line 34 to Column 7, line 16 (relevant portions incorporated herein by reference). Parks U.S. Pat. No. 4,295,987 discloses the use of ethylene glycol diacrylate, tetraethyleneglycol diacrylate and methylene-bis-acrylamide as crosslinking agents for polyacrylate based hydrophilic resins (relevant portions incorporated herein by reference). Japanese patent 55-82104 discloses the use of ethylene glycol di(meth)acrylate, trimethylol propane tri(meth)acrylate and propylene glycol di(meth)acrylate as crosslinking agents for polyacrylate based hydrophilic resins (relevant portions incorporated herein by reference). Masuda et al U.S. Pat. No. 4,076,663 discloses a water absorbent resin is produced by polymerizing (A) starch or cellulose, (B) at least one monomer having a polymerizable double bond which is water-soluble or becomes water-soluble by hydrolysis and (C) a crosslinking agent, and subjecting, if necessary, the resulting product to hydrolysis. Disclosed as among useful crosslinking agents are di- or poly-esters of unsaturated mono- or poly-carboxylic acids with polyols such as di- or tri-(meth)acrylic acid esters of polyols (such as ethylene glycol, trimethylol propane, glycerine, polyoxyethylene glycols, polyoxypropylene glycols, and the like (relevant portions incorporated herein by reference).
Crosslinking improves the strength of the hydrophilic resins and improves the retention of absorbed fluids when the hydrophilic resin is placed under a load and reduces the percentage of extractable materials present, but on the other hand results in a lower capacity of the resin and in some areas may require more of the resin to achieve desired absorption capacity. Extractable materials are water soluble oligomers or non-crosslinked polymers which can be extracted from the hydrophilic resins when exposed to aqueous fluids. The presence of extractable materials reduces the efficacy of the water absorbent particles. Many compounds useful as crosslinkers are not water soluble and require the presence of surfactants or dispersants to solubilize or disperse them so they can be present in the reaction medium to crosslink the hydrophilic resins. The presence of such dispersants and surfactants often adversely affect the absorbent properties of the hydrophilic resins.
What are needed are hydrophilic resins which have high absorption capacity, low extractable materials and high toughness or gel modulus. What is further needed is a process for the preparation of such resins. What are needed are crosslinking agents which are water soluble. The properties desired in hydrophilic resin vary depending upon the application and the desired effect of the resins. Thus, what are needed are processes which are flexible in preparing hydrophilic resins with varied properties such as absorption capacity and absorption under load.